Sunday, September 15, 2013

Into the Great Unknown: Vulcan the Fire God says "You Call That Little Piece of Concrete a Dam?"

Dam engineers sure love their dam creations. The Colorado River, being the only river of note draining the Colorado Plateau, was a target of their fevered dreams, and major projects have "tamed" the river, most notably at Hoover Dam/Lake Mead, and Glen Canyon Dam/Lake Powell (above). Those who administer the giant concrete plugs love to cite the statistics: Hoover holds back 28 million acre feet, Powell 24 million, Mead is 112 miles long when full, Powell is 186 miles. It can't be denied that the concrete monsters have had a huge effect on the ecosystems of the river. Glen Canyon is entirely submerged. The river downstream runs cold all year, and surges high and low in response to electrical production needs. Native fish and flora struggle to survive in the new regime.

For me though, the last two days were some of the most astounding because we had reached the site of one of the most extraordinary geological stories in the entire history of the Grand Canyon. Visitors to the main tourist areas on the north and south rims of the canyon never see the rocks that lined the canyon walls around us, and are often surprised to find they exist at all: miles and miles of basaltic lava flows!

The edge of the Colorado Plateau is punctuated by a series of north trending extensional ("normal") fault zones. They represent the boundary zone between the thick crust of the Colorado Plateau, and the thin extended crust of the Basin and Range Province that reaches across Arizona, Nevada and eastern California. When the crust stretches and breaks, pressure is released in the Earth's mantle below, allowing partial melting of the hot pliable rock. The resulting magma follows the fault zones to the surface. Between 1.8 million and just 1,300 years ago, at least 150 eruptions took place in the vicinity of the western Grand Canyon, covering 600 square miles, forming the Uinkaret Volcanic Field.

Most importantly, at least 13 of these flows spilled over the edge of the canyon and filled the canyon bottom. Vulcan, the fire god, had built his own version of Bureau of Reclamation dams. They weren't small dams. They were hundreds of feet high, and one topped out at least 2,500 feet (Glen Canyon Dam is 710 feet tall). It was the remnants of these lava flows and lava dams that surrounded us as we floated down the river. It was the first time I had seen these rocks. I was mesmerized (yes, we geologists are a strange lot).

What's even more incredible are the lakes that formed behind the dam. The largest dam formed a lake that backed the river up into Utah. If it happened today, the lava dam lake would inundate Lake Powell. It would make for a long hard rafting journey, but the rapid at the end would have been memorable...

Even more mysterious would be how the lakes met their end. It's still the subject of some research, but evidence suggests that at least five of the lava dams failed catastrophically, collapsing and ending the lake in days rather than years. What kind of evidence? The most compelling would be river deposits containing basalt boulders 115 feet across. How do you move boulders that big?

The amount of water unleashed on the lower canyon by such a failure is almost
unimaginable. A
modest 'fake' flood produced by releasing water from Glen Canyon Dam up
the river might involve flows of 40-45,000 cubic feet per second. The
largest historically recorded flood (in 1884) produced flows of about
300,000 cfs. A researcher has found evidence of a flood of 400,000 cfs
around 4,000 years ago. Estimates of major floods during the
Pleistocene ice ages range in the vicinity of a million cubic feet per
second.

The collapse of a 1,500 foot tall lava dam may have produced a flood of 15 million cubic feet per second. That's more than 30 times larger than the biggest flood ever recorded on the Mississippi River. That's how you move 115 foot boulders.

Pictures of gigantic floods filled my imagination as we drifted past lava flow after lava flow. At first, the most vivid outcrops were the flows that had spilled over the rim in the vicinity of Lava Falls and Whitmore Wash. As we floated downstream, the basalt flows tracked along the river, forming low cliffs that went on for miles. The longest flows traveled more than sixty miles down the river bed.

In places the lava flows were thick enough to develop columnar jointing, similar to places like Devils Postpile in California or the Giant's Causeway in Ireland. The columns form when the lava flow pools and then contracts while it cools. The contraction causes the vertical fractures to develop, and they characteristically form hexagonal columns or sometimes rosettes. All in all, the day had been fascinating.

Lava wasn't the only feature of the day. At Whitmore Wash we had a chance to hike up to some interesting pictographs on a sandstone panel a few hundred feet above the river.

The view up the river was fantastic...

We camped at the very creatively named 202 Mile Camp. While I was cooking, my nephew came up to report that the bank of the river was collapsing. I wandered down to have a look and found that an underwater slide was causing large slabs of sand to be pulled towards the river, forming a large arcuate landslide scarp. After seeing a gigantic normal fault to begin the day, it was interesting to see a small-scale version of the same kind of faulting along the riverbank. The little collapsed block in the center would be called a graben.

The slide ultimately ate up a lot of the shoreline, more than 30 feet, and it was clear that a lot of sand was being lost to the deeper part of the river channel. Ever since the floodgates of Glen Canyon Dam closed in 1963, sand has been disappearing along all the shorelines of the river. There have been a few attempts to produce artificial floods that have temporarily moved sand back onto the beaches, but without the sand that is now trapped in Lake Powell, the beaches are going to continue to disappear.

At the end of the day, the moon made an appearance. It was the first we had seen of it on pretty much the entire trip. I had enjoyed seeing the Milky Way each night, and the moon would have obscured many of the stars, but it was nice to see the beautiful crescent setting over the basalt cliffs.

With the last of the twilight, I hit the sack, realizing we were down to our final two days on the river. Our takeout at Diamond Creek was only 24 miles downstream.

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About Me

I am a teacher of geology at Modesto Junior College and former president of the National Association of Geoscience Teachers, Far Western Section. I have led field trips all over the western United States, and a few excursions overseas, but my homebase is the Sierra Nevada, the Great Valley, and the Coast Ranges of California.